1. Field of the Invention
This invention relates to a motor of the permanent-magnetic rotor type, and more particularly to a small-sized motor having a rotor equipped with permanent magnets.
2. Description of the Prior Art
A conventional brushless motor which has been called a hall motor or a transistor motor in the art falls under the category of such a motor of the permanent-magnet rotor type as mentioned above.
Such a motor includes a stator having a plurality of main poles arranged so as to be spaced from each other in the circumferential direction thereof and each having a field winding mounted thereon, and a plurality of complementary poles each arranged between the adjacent two main poles so as to be spaced at an interval from the main poles. The motor also includes a rotor, which has a plurality of magnets and is adapted to be stopped at a position of allowing the magnetic reluctance of a gap defined between the stator and the rotor to be balanced. However, the rotor is often stopped at a position of causing the rotor attractive force to the rotor to overcome the circumferential force or torque thereof. The stopping of the rotor at such position causes a problem of rendering the restart of the rotor difficult or substantially impossible.
In order to eliminate such disadvantage, the conventional motor of such type is provided with a construction which allows the rotor to be always started.
More particularly, for example, in one of the conventional motors of the type that a rotor is adapted to be rotated at the inside of a stator, the rotor is constructed to have regions magnetized to allow the north and south poles to alternately appear on the outer periphery of the rotor and unmagnetized regions alternately arranged in turn in the circumferential direction of the rotor. Due to such unmagnetized regions, the rotor is adapted to permit the torque to overcome the attractive force.
Whereas, in one of the conventional motors of the type that a rotor is rotated at the outside of a stator, the rotor is integrally formed by laying a first magnet group consisting of cylindrically arranged magnets corresponding in number to main poles of the stator and a second magnet group consisting of cylindrically arranged magnets twice as many as the main poles on one another, and the first and second magnet groups are joined to each other in a manner such that a half of the joint surface of each of the magnets forming the second magnet group coincides with the joint surface of each magnet of the first magnet group; so that the distribution of torque generated by the second magnet group while the rotor is stopped may have harmonics twice that generated by the second magnet group. The combined torque is adapted to overcome the attractive force as in the rotor having the unmagnetized regions in the aforesaid motor. More particularly, the torque is adapted to constantly overcome the attractive force at the start due to the second magnet group, resulting in the starting torque being generated.
Nevertheless, such conventional motors as described above still have the following defects. The former construction in which the rotor is formed with the unmagnetized regions has a disadvantage of decreasing in operating efficiency of a small-sized motor due to the decrease in utilization of the magnets as compared with a motor which does not have such an unmagnetized region. In the latter construction, it is required to combine the first and second magnet groups to form the rotor and align the joint surface of each magnet of the former magnet group with that of the corresponding magnet of the latter magnet group, resulting in the manufacturing process being complicated to thereby increase the manufacturing cost of a small-sized motor.